In situ combustion process



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A? TORNEYS United States Patent 2,994,374 IN SITU COMBUSTION PROCESSFrancis W. Crawford and Joseph C. Trantham, Bartlesville, Ckla.,assignors to Phillips Petroleum Company, a corporation of Delaware FiledOct. 28, 1957, Ser. No. 692,857 13 Claims. (Cl. 166-11) This inventionrelates to a process for initiating in situ combustion in a stratumcontaining carbonaceous material and to a process for the recovery ofhydrocarbons from such a stratum.

ln situ combustion in the recovery of hydrocarbons from undergroundstrata containing carbonaceous material is becoming more prevalent inthe petroleum industry. In this technique of production, combustion isinitiated in the carbonaceous stratum and the resulting combustion zoneis caused to move thru the stratum by either inverse or direct air drivewhereby the heat of combustion of a substantial proportion of thehydrocarbon in the stratum drives out and usually upgrades a substantialproportion of the remaining hydrocarbon material. One of the problemsinvolved in the recovery of hydrocarbons by in situ combustion lies inthe ditculty in initiating combustion of the carbonaceous deposit in thestratum and establishing a combustion zone of substantial extent.Usually the section of a stratum adjacent an ignition borehole is heatedfor an extended period, such as several days or even weeks, so as toraise the temperature of the section of stratum to a combustionsupporting temperature for the carbonaceous material therein and, whilethe stratum is at this temperature, air or other oxygen containing gasis injected into the stratum so as to initiate combustion of thecarbonaceous deposit.

In starting in situ combustion of oil and tar sands, etc., there hasbeen considerable difiiculty in establishing a self-sustainingcombustion at the producing well bore by inverse injection because thehigh temperatures developed destroy the heaters and other well equipmentbefore the combustion front becomes sufficiently well established topropagate into the formation.

An object of the invention is to provide an improved process for theproduction of hydrocarbons by in situ combustion in a stratum containingcarbonaceous material. Another object is to provide an improved processfor initiating combustion in a carbonaceous stratum. A further object isto provide a process for initiating in situ combustion in a carbonaceousstratum which avoids the use of a heater in the ignition borehole. Otherobjects of the invention will become apparent upon consideration of theaccompanying disclosure.

The broadest aspect of the invention comprises passing a combustiblemixture of fuel and free oxygen-containing gas thru a carbonaceousstratum into an ignition borehole therein and igniting the combustiblemixture as it enters the -borehole so that the mixture is burned on thesurface of the borehole within the carbonaceous stratum, thereby heatingthe same to ignition temperature when free-oxygen-containing gas ispresent and, while the stratum is at ignition temperature, injectingfree-oxygen containing gas into the stratum so as to ignite the hotcarboneceous material and thereby initiate in situ combustion therein.In order to effect this type of ignition technique, injection boreholesclosely surrounding the ignition borehole are drilled in the stratum andthese are spaced in the range of about 6" to several feet from theignition borehole. After the stratum around the ignition borehole israised in temperature to about 500 to 600 F., or higher, by burning thecombustible mixture in the ignition borehole and in the formationsurrounding the same, in situ combustion of the carbonaceous material isreadily Patented Aug. 1, 1961 initiated by contacting the hot materialwith air, or air enriched with oxygen. In accordance with one embodimentof the invention, the concentration of oxygen in the combustible mixtureis increased so that excess oxygen over the stoichiometric amountrequired for combustion of the injected fuel is present in the injectedgas so that ignition of the carbonaceous material gradually takes placeand, as the combustion of carbonaceous material increases in rate, theflow of fue] gas or the concentration of this component in thecombustible mixture is decreased and cut olf entirely so that thecombustion feeds entirely on the carbonaceous material in the stratum.It is also feasible to maintain the flow of fuel gas constant andgradually increase the flow of air until the combustion of carbonaceousmaterial is well established before cutting olf the ow of fuel gas.

In accordance with another embodiment of the invention, ignition of thehot carbonaceous material is initiated by direct injection of air orother free-oxygen-containing gas into the ignition borehole aftercutting olf the flow of combustible gas thru the stratum. In this methodof operating, combustion gas is removed thru the injection boreholesuntil the combustion zone approaches the vicinity of the injectionboreholes. Direct injection of air may be continued so as to drive thecombustion zone beyond the injection boreholes by sealing the injectionboreholes and opening more remote boreholes spaced farther from theignition borehole than the injection boreholes, in which case the remoteboreholes function as producing wells from which the product hydrocarbonare produced by conventional methods.

A preferred method of operation comprises injecting air thru theinjection boreholes until the combustion front approaches the points ofair injection and, thereafter, sealing the injection boreholes andutilizing the remote boreholes as injection boreholes so that thecombustion zone is moved thru the stratum countercurrently to the flowof air until it arrives at the remote boreholes. At this time, withcontinued injection of air, the combustion zone reverses itself indirection and moves back thru the stratum, the combustion feeding on thecarbonized residue remaining in the formation after the rst (inverse)combustion wave passes thru the stratum. This second combustion wave isknown as reflection burning and is described and claimed in the U.S.application of J. C. Trantham et al., Serial Number 529,916.

A 'more complete understanding of the invention may be had by referenceto the accompanying drawing of which FIGURE 1 is a plan view of anarrangement of boreholes or wells suitable for effecting the invention;and FIGURE 2 is a section thru a stratum showing a suitable arrangementof equipment for effecting the invention.

Referring to FIGURE 1, an ignition borehole 10 is surrounded by fourclosely spaced injection boreholes 12 in a typical S-spot pattern andmore remote boreholes 14 are spaced around the inner boreholes in asimilar pattern.

In FIGURE 2, borehole 10 is provided with a casing 16 and tubing orconduit 18 for injection or withdrawal of fiuids from the borehole.Boreholes 12 which are spaced from borehole 10 a distance in the rangeof about 6 to several feet are each provided with casings 20 and atubing 22. An additional conduit 24 connects with conduit or tubing 22above the well head, however, this conduit may connect with casing 20.Fuel is introduced thru conduit 24 and air thru conduit 22 so that thesegases are mixed within conduit 22 and are then passed to the hydrocarbonbearing stratum. Alternatively, the air may be injected thru conduit 2Aand the fuel thru tubing 22. Remote boreholes or wells 14 are providedwith tubing 26 for passage of iuid to and from the producing stratum ashereinafter described. All of the boreholes extend thru stratum 28 whichcontains carbonaceous material, such aS petroleum, tar, or othercarbonaceous deposit to lbe produced in the process. Boreholes 14 may bespaced a distance from borehole in the range of about 40 to 50 feet to1A or 1%; of a mile depending upon the permeability of the stratum andcharacter of the carbonaceous deposit.

In producing hydrocarbons from stratum 28 in accordance with the processof the invention a combustible mixture of fuel gas and air is introducedto permeable stratum 28 thru the conduits leading into boreholes 12,boreholes 14 being sealed and `borehole 10 being open so that thecombustible mixture passes thru the narrow section of stratum 28 betweenboreholes 12 and borehole 10. The combustible mixture is ignited inborehole 10 as it emerges from the stratum by an electric spark. asquib, a flame or pilot light, or by any other suitable means. The rateof ow of the combustible mixture is adjusted so as to cause the burningto take place on the wall of the borehole .in stratum 28. When thesection of stratum adjacent the borehole becomes sutliciently hot,ameless combustion of the mixture takes place within the permeablestratum and therefore heats the stratum to a substantial depth. When thetemperature of the stratum in a section thereof around borehole 10reaches combustionsupporting temperature of the carbonaceous material inthe stratum in the presence of air, the flow of fuel is cut off so thatthe hot formation is contacted with free oxygen, thereby igniting thehot carbonaceous material and causing the resulting combustion front 30to move thru stratum 28 toward boreholes 12 in an annular andcylindrical conformation around borehole 10. As the combustion zone 30approaches boreholes 12, ow of air thru these boreholes is terminated,and air is injected into boreholes 14 thru conduits 26, the air thusinjected passing thru stratum 28 to the hot zone 30 in which combustionhas temporarily ceased. During movement of the combustion front from thearea of boreholes 12 to the area of boreholes 14, production can berecovered thru boreholes 12 or thru boreholes 10 by sealing oil?boreholes 12. As the air again arrives in the hot section of thestratum, combustion is again initiated because the ternperature of thehot zone is above the ignition temperature of the carbonaceous materialin the stratum and now the re-established combustion zone is movedcountercur- -rently to the flow of air toward the remote injectionboreholes 14. I

It is also feasible to utilize direct injection of air thru conduit 18in borehole 10 to initiate the combustion of the stratum zone aroundignition borehole 10, after heating the stratum as previously discussed,and to move the combustion zone 30 toward boreholes 12. In formations orstrata which are not susceptible to plugging during direct injection ofair, the combustion zone 30 may be driven thru the stratum to boreholes14 by injection of air thru borehole 10. In this type of operationboreholes 14 serve as production boreholes with the producedhydrocarbons being recovered by conventional methods thru tubing 26.However, movement of the combustion zone 30 thru the stratum `by inverseair injection thru boreholes 14 is preferred and, during this type ofoperation, borehole 10 functions as a producing borehole with theproduced hydrocarbons being recovered thru tubing 18.

In field operation it has been found necessary to inject air at aminimum rate of 50 s.c.f.h. per square foot of vertical cross section ofstratum in order to maintain and advance the combustion zone in acarbonaceous stratum. For initiation of combustion around an ignitionborehole, the area of the wall of the borehole is considered to be theinitial area and the area to be considered thereafter is the area of theadvancing combustion front, which is of course expanding in a successfuldrive. The upper limit for eicient operation has never been reached dueto the restrictive effect of low permeability of the stratum andcompressor limitations. However, flow rates of 300 to 400 s.c.f.h. persquare foot of area are effective in maintaining and driving thecombustion front thru more permeable strata.

Because of the high temperature in borehole 10 during ignition and theearly phases of the combustion process, it is advisable to installtubing 18 in this ignition borehole after the temperature in theproducing stratum adjacent the borehole has dropped so that damage tothe equipment is avoided.

lIn carbonaceous strata of low permeability, fracturing and propping inconventional manner may be practiced in order to improve thepermeability to gas flow therethru. In many strata the permeability issuiciently high that suitable gas flow rates result from injection gaspressures within the capacity of conventional compressors.

Certain modifications of the invention will become apparent to thoseskilled in the art and the illustrative details disclosed are not to beconstrued as imposing unnecessary limitations on the invention.

We claim:

l. A process for initiating in situ combustion in a stratum containingcombustible carbonaceous material which comprises injecting acombustible gaseous mixture containing free-oxygen and fuel into saidstratum thru a ring of boreholes therein surrounding a central ignitionborehole therein, said ring of boreholes being a distance in the rangeof 6 inches to several feet from said ignition borehole so as to passsaid mixture thru said stratum to said ignition borehole and permeate acomplete annulus of said stratum around said ignition borehole; ignitingsaid mixture as it enters said ignition borehole so as to burn same'adjacent the wall thereof; continuing injection and burning so as toburn said mixture in said annulus and heat said annulus to the ignitiontemperature of said carbonaceous material in free-oxygen-containing gas;while said annulus is at said temperature, igniting the hot carbonaceousmaterial therein by contacting same with free-oxygen-containing gas.

2. The process of claim l wherein the flow of fuel into said stratum isterminated and the ow of free-oxygencontaining gas is continued so as toignite said carbonaceous material.

3. The process of claim 1 wherein flow of fuel is diminished therebyincreasing the concentration of freeoxygen in said mixture so as toignite said carbonaceous material.

4. The process of claim 1 wherein the concentration offree-oxygen-containing gas in said mixture is increased to ignite saidcarbonaceous material.

5. The process of claim 1 including the steps of terminating the flow offree-oxygen containing gas thru said ring of boreholes after in situcombustion has been established and injecting free-oxygen-containing gasinto said stratum thru a more remote borehole so as to pass same to thecombustion zone and advance said zone countercurrently to the ow of saidgas.

6. The process of claim 1 wherein air is injected into said stratum thrusaid ignition borehole after heating said annulus to combustionsupporting temperature to ignite said carbonaceous material and theresulting annulus combustion zone is driven radially outwardly from saidignition borehole.

7. The process of claim 1 wherein air is injected thru said ignitionborehole to ignite said carbonaceous material.

8. A process for producing hydrocarbons from a stratum containingcombustible carbonaceous material which comprises injecting acombustible mixture of fuel gas and free-oxygen-containing gas into saidstratum thru a plurality of injection boreholes surrounding an ignitionborehole and spaced therefrom a distance in the range of 6 inches toseveral feet so as to cause said mixture to ow to said ignitionborehole; igniting said mixture as it enters said ignition borehole soas to burn same adjacent the wall thereof; continuing the injection andburning so as to cause burning of said mixture in an annulus of saidstratum around said ignition borehole, thereby heating aannam saidannulus to the ignition temperature of said carbon- -aceous material insaid free-oxygen-containing gas; while said annulus is at saidtemperature, igniting the hot carbonaceous material therein by injectingfree-oxygen-containing gas into same and continuing the injection so asto extend the resulting combustion zone radially outwardly from saidignition borehole to the area of said injection boreholes; thereafterpassing free-oxygenontaining gas thru said stratum to the advancedcombustion zone from a plurality of boreholes more remote from saidignition borehole than said injection boreholes; and recoveringhydrocarbons driven from said stratum by said combustion.

9. The process of claim 8 wherein said free-oxygen-containing gascomprises air.

10. The process of claim 8 wherein the ignition of said carbonaceousmaterial is effected by injecting air into said stratum thru saidinjection boreholes and produced hydrocarbons are recovered thru saidignition borehole.

11. The process of claim 8 wherein the ignition of the carbonaceousmaterial in said annulus is effected by injecting air into said annulusthru said ignition borehole after heating said annulus to combustionsupporting temperature.

12. A process for producing hydrocarbons from a stratum containingcombustible carbonaceous material which comprises injecting acombustible mixture of fuel gas and free-oxygen-containing gas into saidstratum thru a plurality of injection boreholes surrounding an ignitionborehole and spaced therefrom a distance in the range o inches toseveral feet so as to cause said mixture to to said ignition borehole;igniting said mixture as it ent said ignition borehole so as to yburnsame adjacent wall thereof; continuing the injection and burning u: asubstantial section of said stratum around said ignit borehole is heatedto the ignition temperature of s carbonaceous material in saidfree-oxygen-containing j and said mixture is burning in an annulus ofsaid strati 10 around said ignition borehole; while said annulus is saidtemperature, igniting the hot carbonaceous mater therein by injectingfree-oxygen-containing gas into sal thru said ignition borehole;continuing the injection said gas thru said ignition borehole so as todrive 1 resulting combustion zone thru said stratum to a plural ofsurrounding boreholes beyond said injection borehol and recoveringhydrocarbons driven from said strati by the combustion thru the remoteboreholes.

13. The process of claim 12 wherein said free-oxyge containing gascomprises air.

References Cited in the tile of this patent UNITED STATES PATENTS2,642,943 Smith June 23, 19: 2,793,696 Morse May 28, 191 2,880,803Parker Apr. 7, 19!

